RF power amplifiers are used in a variety of applications such as base stations for wireless communication systems, etc. The signals amplified by the RF power amplifiers often include signals that have a high frequency modulated carrier having frequencies in the 400 megahertz (MHz) to 60 gigahertz (GHz) range. The baseband signal that modulates the carrier is typically at a relatively lower frequency and, depending on the application, can be up to 300 MHz or higher. Many RF power amplifier designs utilize a semiconductor switching device as the amplification device. Examples of these switching devices include power transistor devices, such as a MOSFET (metal-oxide semiconductor field-effect transistor), a DMOS (double-diffused metal-oxide semiconductor) transistor, a GaN HEMT (gallium nitride high electron mobility transistor), a GaN MESFET (gallium nitride metal-semiconductor field-effect transistor), an LDMOS transistor, etc.
Class F amplifier configurations are gaining increased favor in modern RF applications due to their highly efficient operation. In class F operation, the input of the switching device (e.g., the gate) is modulated while a reference terminal of the switching device (e.g., the source) is maintained at a fixed potential. During the ON state of the switching device, voltage is ideally at zero across the output of the switching device while a sinusoidal switching current is present across the output terminal of the switching device. During the OFF state of the switching device, current is ideally at zero across the output of the switching device while a half square wave voltage appears at the output terminal of the switching device. Theoretically, no power is dissipated because both states are characterized by zero IV. In practice, power dissipation occurs at the transition between ON and OFF states when there is an overlap between the current sine wave and the voltage square wave and hence current and voltage simultaneously appear at the output terminal. Highly efficient class F operation is obtained by minimizing this overlap.
One technique for minimizing the current-voltage overlap in class F amplifiers involves harmonic filtering. By mitigating harmonic oscillation at the output of the device, the shapes the voltage and current waveforms are beneficially controlled for minimal overlap. For optimum efficiency, the output of the amplifier should include a short circuit path for even higher ordered harmonics (e.g., 2F0, 4F0, 6F0, etc) of the fundamental frequency F0 of the RF signal being amplified.
Known techniques for harmonic tuning of Class F amplifiers include incorporating filters into the impedance matching networks that are connected to the input and output terminals of the amplifier device. These impedance matching networks can be provided on a printed circuit board (PCB) that accommodates the packaged amplifier device. Alternatively, or in addition, harmonic filters can be incorporated into the packaged amplifier device itself by placing discrete capacitors and inductors between the integrated circuit and the package leads.
Designers face several notable challenges with respect to tuning circuits for filtering higher ordered harmonics. For example, parasitic effects predominate at the higher frequencies that higher ordered harmonics occur at. Thus, as the filtering circuitry is separated from the current source by elements that contain non-negligible parasitic inductance, capacitance, etc., the ability to effectively tune out higher order harmonics becomes more difficult. Moreover, the inclusion of separate turning networks for higher order harmonics increases the size, cost and component count of the device.